There is an increasing requirement for battery systems which are intended to be used in stationary applications such as wind power installations and emergency power supply systems, or else in vehicles. All of these requirements place stringent demands on the reliability and fail-safety. This is because complete failure of the voltage supply by the battery system can lead to a failure of the overall system. For example, in the case of wind power installations, batteries are used in order to adjust the rotor blades when the wind is strong, and thus to protect the installation against excessive mechanical loads which may damage or even destroy the wind power installation. In the event of the failure of the battery in an electric car, the latter would become impossible to drive. In turn, an emergency power supply system is actually intended to ensure interruption-free operation, for example of a hospital, and therefore, as far as possible, cannot itself fail.
In order to allow the power and energy required for the respective application to be made available, individual battery cells are connected in series, and in some cases additionally in parallel. In batteries for practical applications, a multiplicity of battery cells are connected in series in order to achieve the high operating voltage, which is required for example in a motor vehicle with an electric drive motor for the drive motor, by addition of the voltages of the individual cells. Since the total output current from the battery flows in each of the battery cells, because the battery cells are connected in series, with the charge transport taking place by means of electrochemical processes within the battery cells, the failure of a single battery cell in the extreme means that the overall arrangement can no longer provide any current, and therefore no electrical energy. Furthermore, this means that it is impossible to operate the individual battery cells matched to their respective state (state of charge, temperature, aging). A further disadvantage of pure series connection of battery cells is that the output voltage of the battery depends on the state of charge of the battery cells, and is thus variable. However, it would be a major advantage if the battery were to supply an output voltage which is as constant as possible, since the following power-electronic system components, such as inverters or drive motors can then be designed to be considerably more cost-effective, and more efficient, for this output voltage.